A gas turbine engine often includes a secondary flow system, which extracts bleed air from the engine's core airflow path for turbine cooling, seal buffering, and/or other purposes. By common design, a secondary flow system extracts bleed air from one or more locations along the compressor section of the engine. When utilized to cool a high pressure turbine or another hot section component, the temperature of the bleed air extracted from the compressor section is ideally as low as practical. At the same time, the pressure of the bleed air should be sufficiently high to support an adequate flow rate through the system's cooling circuits. When the airflow is extracted from a compressor section containing a centrifugal compressor or “impeller,” these competing criteria are often best satisfied by extracting bleed air from an intermediate or middle section of the impeller. Mid-Impeller Bleed (MIB) systems have thus been developed, which extract bleed air from the impeller mid section during engine operation. In certain cases, the MIB system can include flow passages formed in the impeller itself, which direct bleed air radially inward to cooling circuits extending along the shaft or shafts of the engine. Alternatively, the MIB system may include holes or ports formed in the static structure surrounding the impeller, such as an impeller shroud, through which bleed air is extracted and delivered to cooling circuits running radially outboard of the compressor and combustor sections. Impeller shrouds including such MIB ports are referred to herein as “MIB impeller shrouds.”
By extracting bleed air from an impeller's mid section, a well-designed MIB system can boost the efficiency of the secondary flow system and improve overall gas turbine engine performance. Existing MIB systems and MIB impeller shroud designs remain limited in certain respects, however. Conventional MIB impeller shrouds may introduce undesired inefficiencies into the MIB system by, for example, permitting relatively high pressure losses as airflow is extracted from the rotating impeller through the static impeller shroud. Such pressure losses decrease the efficiency with which the velocity imparted to the compressed airflow by the rotating impeller is converted to static pressure within the MIB plenum. As a further drawback, conventional MIB impeller shrouds can be undesirably bulky, heavy, and costly to produce. Moreover, conventional MIB impeller shrouds and the manner in which such shrouds are secured within the engine compressor section may require the provision of relatively large running clearances between the impeller blade tips and interior surface of the shroud to ensure that physical contact does not occur between these components. Such large running clearances further negatively affect the compressor section efficiency and detract from overall gas turbine engine performance.
It is thus desirable to provide a MIB impeller shroud or a structure including a MIB impeller shroud (referred to herein as an “impeller shroud support”), which overcomes one or more of the aforementioned limitations. Ideally, embodiments of an impeller shroud support would enable airflow to be extracted from an impeller in a highly efficient manner that minimizes pressure losses, while further allowing a reduction in running clearances between the shroud and the impeller blade tips. It would also be desirable for such an impeller shroud support to be relatively lightweight, structurally robust, and readily manufacturable. Finally, it would be desirable to provide embodiments of a gas turbine engine or other turbomachine including impeller shroud supports having such characteristics. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.